1. Field of the Invention
The present invention relates to a method for the production of tungsten carbide based hard metal tools or components using the powder injection moulding or extrusion method and a binder system therefore.
2. Description of the Related Art
Hard metals based on tungsten carbide are composites consisting of small (μm-scale) grains of at least one hard phase in a binder phase. These materials always contain the hard phase tungsten carbide (WC). In addition, other metal carbides with the general composition (Ti,Nb,Ta,W)C may also be included, as well as metal carbonitrides, e.g., Ti(C,N). The binder phase usually consists of cobalt (Co). Other binder phase compositions may also be used, e.g., combinations of Co, Ni, and Fe, or Ni and Fe.
Industrial production of tungsten carbide based hard metals often includes blending of given proportions of powders of raw materials and additives in the wet state using a milling liquid. This liquid is often an alcohol, e.g., ethanol or water or a mixture thereof. The mixture is then milled into homogeneous slurry. The wet milling operation is made with the purpose of deagglomerating and mixing the raw materials intimately. Individual raw material grains are also disintegrated to some extent. The obtained slurry is then dried and granulated, e.g. by means of a spray dryer. The granulate thus obtained may then be used in uni-axial pressing of green bodies or for extrusion or injection moulding.
Injection moulding is common in the plastics industry, where material containing thermoplastics or thermosetting polymers are heated and forced into a mould with the desired shape. The method is often referred to as Powder Injection Moulding (PIM) when used in powder technology. The method is preferably used for parts with complex geometry. In powder injection moulding of tungsten carbide based hard metal parts, four consecutive steps are applied:
1. Mixing of the granulated cemented carbide powder with a binder system. The binder system acts as a carrier for the powder and constitutes 25-60 volume % of the resulting material, often referred to as the feedstock. The exact concentration is dependent on the desired process properties during moulding. The mixing is made with all organic constituents in molten state. The resulting feedstock is obtained as pellets of approximate size 4×4 mm.
2. Injection moulding is performed using the mixed feedstock. The material is heated to 100-240° C. and then forced into a cavity with the desired shape. The thus obtained part is cooled and then removed from the cavity.
3. Removing the binder from the obtained part. The removal can be obtained by extracting of the parts in a suitable solvent and/or by heating in a furnace with a suitable atmosphere. This step is often referred to as the debinding step.
4. Sintering of the parts. Common sintering procedures for cemented carbides are applied.
Extrusion of the feedstock comprises steps 1, 3 and 4 above. Instead of forcing the feedstock into a cavity of the desired shape, the feedstock is continuously forced through a die with the desired cross section.
The solids loading, φ, of the feedstock is the volumetric amount of hard constituents, compared to the organic constituents. φ can be calculated using the following equation:
  ϕ  =                    ρ        f            -              ρ        v                            ρ        s            -              ρ        v            where ρs is the density of the cemented carbide as sintered, ρv is the mean density of the organic constituents and ρf is the density of the feedstock, measured with a helium pycnometer.
In the case of having a low solids loading of the feedstock, φ, problems with cracks, voids, blisters and distorted parts may occur. In the case of having a high solids loading, φ, problems with mould filling, extended mould wear, weld lines, which may open during sintering, forming cracks and surface defects due to too high viscosity as well as mould release problems may occur.